Flat single layer membranes have been used as actuator structures in micro-electro-mechanical systems ("MEMS"). Some MEMS devices that incorporate actuator structures include microvalves, which are turned on or off by the actuator and micropumps that are actuated to pump by the actuator.
Previously, flat single layer silicon membranes have been used in actuator structures. However, silicon has a relatively high Young's modulus, e.g. 139 to 190 GPa, which can limit the actuator's deflection capability. Typically, to achieve about a 50 .mu.m deflection, a 5 mm.times.5 mm.times.50 .mu.m single layer silicon membrane is usually needed.
For MEMS applications, smaller membranes that occupy less chip real estate is desired. However, flat single layer membranes with smaller dimensions achieve smaller deflections. Larger deflections are desirable for facilitating high flow applications in some microvalve implementations. In high flow microvalves, a large distance between the valve seat and the rest position of the actuator membrane is desired. This large distance allows a large amount of flow to pass through when the microvalve is opened. Hence, to close the microvalve, a large deflection distance is required from the actuator membrane. Thus, in some MEMS applications, both large deflections and small size are desired.
There are different approaches to fabricating a MEMS applicable actuator membrane capable of achieving large deflections. One approach is to make a thinner membrane. Thinner membranes may provide increased deflections because the thinner membrane can stretch more during actuation. However, thinner membranes may have a lower pressure tolerance before the membrane loses structural integrity and bursts.
Another approach is to use flexible materials for the membrane. Low Young's modulus materials with high elongation, good compatibility with IC processes, and good sealing properties on rough surfaces are generally desired. One such material is silicone rubber, which has a Young's modulus of approximately 1 MPa, as described in U.S. patent application Ser. No. 60/036,253, filed Jan. 24, 1997 the disclosure of which is incorporated by reference.
Yet, another approach is to modify the structure of the MEMS actuator membrane in order to maximize deflection while minimizing size.